1. Field of the Invention
The invention relates to an apparatus and method that provide positive mechanical control from the surface of a well, or from a drill ship or platform, of the release of plugs used during a cementing operation of a subterranean well.
2. Description of the Prior Art
As a step in the completion operation of a subterranean well, casing is run into the well and the annular area exterior of the casing and within the open bore thereafter is cemented to secure the casing within the well. Cementing plugs are utilized in the cementing operation and are run ahead of the cement slurry in order to wipe mud off the walls of the casing and to prevent cement from being contaminated with the drilling fluid previously circulated within the casing and the well. Such plugs are oftentimes run into the well within the casing and behind a cement slurry in order to close off check valves, open stage collars, and reclose stage collars during multi-stage cementing operations.
When subsea wells are completed utilizing drill ships or floating platforms, it is difficult to tie back the casing string to the surface of the platform or drill ship because of the motion between the platform or drill ship and the subsea well head. Typically, cementing operations under such conditions have incorporated cementing plugs which have been affixed to the bottom of the drill string by shear pins, collet releasing mechanisms, or the like. After the drill string is landed in the well head, the cement plugs are released from engaged position on the drill string by dropping or pumping balls, darts, and the like, to hydraulically activate the release of the plugs. The drill pipe containing the secured cementing plugs is run on a drill pipe having a conventional expansion joint to longitudinally compensate for the movement of the drill ship on the ocean, and the drill pipe is tied back to the well head. Now, because the drill pipe is tied back, a full size cementing plug cannot be pumped or inserted through the drill pipe.
Regardless of whether the cementing operation is performed on an off-shore or inland well, most prior art plug dropping mechanisms are not completely reliable for efficient sequential release of the plugs, because of the use of hydraulic activation means to disengage the plug from its secured position, prior to pumping the plug downhole.
The present invention provides an apparatus and method for release of plugs without the use of auxiliary pumpable means, such as balls, darts, shear pins and the like. The present invention incorporates an apparatus which sequentially releases the plugs from their respective heads in a sequential, highly reliable, operation which is dependent only upon rotational manipulation of the drill pipe or other conduit to release the plugs from the engaged position relative to the pipe or conduit. When utilized above an off-shore well, the apparatus provides means for release of the plugs above the ocean floor and above the well head, and thus further provides a more reliable means of releasing plugs.